Semiconductor device and power amplifier module

ABSTRACT

A circuit element is formed on a substrate made of a compound semiconductor. A bonding pad is disposed on the circuit element so as to at least partially overlap the circuit element. The bonding pad includes a first metal film and a second metal film formed on the first metal film. A metal material of the second metal film has a higher Young&#39;s modulus than a metal material of the first metal film.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. patent application Ser. No.15/946,552 filed Apr. 5, 2018, which claims benefit of priority toJapanese Patent Application No. 2017-085831 filed Apr. 25, 2017, theentire content of which is incorporated herein by reference.

BACKGROUND Technical Field

The present disclosure relates to a semiconductor device and a poweramplifier module.

Background Art

Japanese Unexamined Patent Application Publication No. 2005-236259 andInternational Publication No. 2001/018865 each disclose a semiconductordevice for a high-frequency amplifier module using a heterojunctionbipolar transistor (HBT). The semiconductor device includes a protectioncircuit for protecting the HBT. The protection circuit preventsbreakdown of the HBT when an excessive voltage such as staticelectricity is applied to the HBT. The protection circuit has a circuitconfiguration in which a plurality of diodes are connected in series,and is connected between a collector and an emitter of the HBT.

Japanese Unexamined Patent Application Publication No. 2000-164623discloses a semiconductor device in which an electrode pad is formed ona wiring line or an active element. A projection electrode forprotecting the wiring line or the active element during bonding isformed on a surface of the electrode pad (bonding pad). The projectionelectrode is formed by substituting Al on the surface of the electrodepad with Zn, which is capable of being subjected to substitutionreaction with Ni, and then coating the surface of the electrode pad withNiP by electroless plating. It is also disclosed that, besides NiP, aCu-based metal which can be deposited by electroless plating may beused.

SUMMARY

In the semiconductor devices disclosed in Japanese Unexamined PatentApplication Publication No. 2005-236259 and International PublicationNo. 2001/018865, it is necessary to secure, on a semiconductorsubstrate, a region for arranging the plurality of diodes thatconstitute the protective circuit. As a result, the chip area becomeslarger. An increase in the chip area makes it difficult to realize areduction in the cost of compound semiconductor devices.

As in the semiconductor device disclosed in Japanese Unexamined PatentApplication Publication No. 2000-164623, the arrangement of the bondingpad on a wiring line or an active element enables an increase in thechip area to be suppressed. In silicon-based semiconductor processes, Alis used in wiring lines and bonding pads. However, in compoundsemiconductor processes, Al is not usually used in wiring lines orbonding pads. Therefore, it is difficult to apply the techniquedisclosed in Japanese Unexamined Patent Application Publication No.2000-164623 to compound semiconductor processes without modifications.

Accordingly, the present disclosure provides a semiconductor device thatincludes a substrate made of a compound semiconductor, in which anincrease in the chip area can be suppressed. The present disclosure alsoprovides a power amplifier module that includes the semiconductordevice.

According to a preferred embodiment of the present disclosure, asemiconductor device includes a circuit element formed on a substratemade of a compound semiconductor, and a bonding pad disposed on thecircuit element so as to at least partially overlap the circuit element.In the semiconductor device, the bonding pad includes a first metal filmand a second metal film formed on the first metal film, and a metalmaterial of the second metal film has a higher Young's modulus than ametal material of the first metal film.

Since the circuit element and the bonding pad are disposed so as topartially overlap, it is not necessary to secure a dedicated region forarranging the bonding pad. With this structure, an increase in the chiparea can be suppressed. The second metal film functions as a stressdispersion sheet to thereby disperse a stress generated in the circuitelement during bonding, and thus breakdown of the circuit element can besuppressed.

The bonding pad may further include a third metal film formed on thesecond metal film, and the third metal film may be formed of the samemetal material as the first metal film. As the third metal film, a metalmaterial having a low contact resistance with a bonding wire can beused. The contact resistance between the bonding pad and the bondingwire can be reduced compared with the case where the bonding wire isdirectly bonded to the second metal film.

The first metal film and the third metal film may be formed of Au, andthe second metal film may include a layer made of at least one metalmaterial selected from the group consisting of Cu, Ni, and Mo. It ispossible to use Au, which is typically used for wiring lines forconnecting a plurality of circuit elements formed on a compoundsemiconductor substrate. Since Cu, Ni, and Mo are harder than Au, thesecond metal film functions as a stress dispersion sheet. The secondmetal film may include two layers made of at least two metal materialsselected from the group consisting of Cu, Ni, and Mo. By combining aplurality of metal materials, it is also possible to enhance the degreeof freedom of selection of hardness, electrical resistivity, and thelike as a whole of the second metal film.

The semiconductor device may further include a protective film on thefirst metal film, in which the protective film has an opening disposedinside the first metal film in plan view, and the second metal film andthe third metal film are disposed inside the opening in plan view. Aload applied to the second metal film and the third metal film duringbonding is not transferred to the protective film. Accordingly, theprotective film is unlikely to be damaged during bonding.

The semiconductor device may further include a fourth metal film that isformed of the same metal material as the first metal film and thatcovers a region of an upper surface of the first metal film inside theopening, the region not being covered with the second metal film, a sidesurface of the second metal film, and a side surface and an uppersurface of the third metal film. Since a bonding interface between thesecond metal film and the third metal film, which are made of dissimilarmetals, is covered with the fourth metal film, occurrence of galvaniccorrosion can be suppressed.

The compound semiconductor of the substrate may have a zincblend crystalstructure, the substrate may have an upper surface having an off-angleof 4° or less from a (100) plane, and the circuit element may be oneelement selected from the group consisting of a heterojunction bipolartransistor, a field-effect transistor, a diode, a capacitor, and aresistive element. It is possible to suppress damage of circuitelements, such as a heterojunction bipolar transistor, a field-effecttransistor, a diode, a capacitor, and a resistive element, which areformed on a compound semiconductor substrate having a zincblend crystalstructure in configurations in which these circuit elements are disposedso as to overlap bonding pads.

The circuit element may include a mesa structure including asemiconductor layer that is epitaxially grown from the substrate, anupper surface of the mesa structure may have a rectangular or squareplanar shape including a side parallel to a [01-1] direction and a sideparallel to a [011] direction, and the side parallel to the [011]direction may be shorter than the side parallel to the [01-1] direction.

When a semiconductor layer is anisotropically etched by wet etchingusing a rectangular or square etching mask including a side parallel tothe [01-1] direction and a side parallel to the [011] direction, sideetching proceeds from the side parallel to the [011] direction,resulting in the formation of an eave-shaped portion. Herein, the term“anisotropic etching” refers to etching performed by using thedifference in the etching rate due to crystal planes. The eave-shapedportion is easily broken by a stress generated during bonding. Since theside parallel to the [011] direction is shorter than the side parallelto the [01-1] direction, a region where such an eave-shaped portion isformed is narrow. Accordingly, the breakage during bonding can besuppressed.

The circuit element may include a mesa structure including asemiconductor layer that is epitaxially grown from the substrate, and anupper surface of the mesa structure may have a polygonal planar shapeincluding a side parallel to a [001] direction and a side parallel to a[010] direction.

When a semiconductor layer is anisotropically etched by wet etchingusing a polygonal etching mask including a side parallel to the [001]direction and a side parallel to the [010] direction, side surfaces thatrise substantially perpendicularly are formed so as to correspond tothese sides. Since no eave-shaped portions are formed, the breakageduring bonding can be suppressed.

The upper surface of the mesa structure may have a parallel hexagonalplanar shape further including a side parallel to a [01-1] direction. Aside surface corresponding to the side parallel to the [01-1] directionis gently inclined. Since no eave-shaped portions are formed, thebreakage during bonding can be suppressed.

According to another preferred embodiment of the present disclosure, apower amplifier module includes a semiconductor device including a poweramplifier circuit that includes a heterojunction bipolar transistorformed on a substrate made of a compound semiconductor, a protectioncircuit connected between an emitter and a collector of theheterojunction bipolar transistor, and a bonding pad functioning as anoutput terminal of the power amplifier circuit, in which at least onecircuit element that forms the power amplifier circuit and theprotection circuit and the bonding pad are disposed so as to partiallyoverlap, the bonding pad includes at least two layers of a first metalfilm and a second metal film, and a metal material of the second metalfilm has a higher Young's modulus than a metal material of the firstmetal film; a printed circuit board on which the semiconductor device ismounted; and a bonding wire that is bonded to the bonding pad to connectthe bonding pad to a wiring line of the printed circuit board.

Since at least one circuit element and the bonding pad are disposed soas to partially overlap, it is not necessary to secure a dedicatedregion for arranging the bonding pad. With this structure, an increasein the chip area of the semiconductor device can be suppressed. Thesecond metal film functions as a stress dispersion sheet to therebydisperse a stress generated in the circuit element during bonding, andthus breakdown of the circuit element can be suppressed.

Since the circuit element and the bonding pad are disposed so as topartially overlap, it is not necessary to secure a dedicated region forarranging the bonding pad. With this structure, an increase in the chiparea can be suppressed. The second metal film functions as a stressdispersion sheet to thereby disperse a stress generated in the circuitelement during bonding, and thus breakdown of the circuit element can besuppressed.

Other features, elements, characteristics and advantages of the presentdisclosure will become more apparent from the following detaileddescription of preferred embodiments of the present disclosure withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of a part of a semiconductor device according toa first embodiment, and FIGS. 1B and 1C are sectional views taken alongdash-dotted lines 1B-1B and 1C-1C of FIG. 1A, respectively;

FIG. 2 is an equivalent circuit diagram of a power amplifier moduleincluding a semiconductor device according to the first embodiment;

FIGS. 3A and 3B are schematic sectional views of a bonding pad of asemiconductor device according to the first embodiment before wirebonding and during wire bonding, respectively, and FIGS. 3C and 3D areschematic sectional views of a bonding pad of a semiconductor deviceaccording to a comparative example before wire bonding and during wirebonding, respectively;

FIG. 4 is a table listing Vickers hardness, Young's modulus, andelectrical resistivity of various metals;

FIG. 5 is a sectional view of a semiconductor device according to asecond embodiment;

FIG. 6 is a sectional view of a semiconductor device according to athird embodiment;

FIGS. 7A and 7B are sectional views of a semiconductor device accordingto the third embodiment during its production;

FIGS. 8A and 8B are sectional views of a semiconductor device accordingto the third embodiment during its production;

FIGS. 9A and 9B are sectional views of a semiconductor device accordingto the third embodiment during its production;

FIGS. 10A and 10B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 11A and 11B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 12A and 12B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 13A and 13B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 14A and 14B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 15A and 15B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 16A and 16B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 17A and 17B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 18A and 18B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 19A and 19B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 20A and 20B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 21A and 21B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 22A and 22B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 23A and 23B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 24A and 24B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 25A and 25B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 26A and 26B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 27A and 27B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 28A and 28B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 29A and 29B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 30A and 30B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIGS. 31A and 31B are sectional views of a semiconductor deviceaccording to the third embodiment during its production;

FIG. 32A is a sectional view of a semiconductor device according to afourth embodiment, and FIGS. 32B and 32C are sectional views ofsemiconductor devices according to modifications of the fourthembodiment;

FIG. 33A is a plan view of a regular octagonal mesa formed, byanisotropic etching, on a GaAs substrate having a (100) plane as anupper surface, and FIGS. 33B, 33C, 33D, and 33E are sectional viewstaken along dash-dotted lines 33B-33B, 33C-33C, 33D-33D, and 33E-33E ofFIG. 33A, respectively;

FIG. 34A is a plan view of one diode of a semiconductor device accordingto a fifth embodiment, and FIGS. 34B and 34C are sectional views takenalong dash-dotted lines 34B-34B and 34C-34C of FIG. 34A, respectively;

FIG. 35A is a plan view of one diode of a semiconductor device accordingto a sixth embodiment, and FIGS. 35B and 35C are sectional views takenalong dash-dotted lines 35B-35B and 35C-35C of FIG. 35A, respectively;

FIG. 36A is a plan view of one diode of a semiconductor device accordingto a seventh embodiment, and FIG. 36B is a sectional view taken alongdash-dotted line 36B-36B of FIG. 36A; and

FIG. 37 is a sectional view of a power amplifier module according to aneighth embodiment.

DETAILED DESCRIPTION First Embodiment

A semiconductor device according to a first embodiment will be describedwith reference to FIGS. 1A to 4.

FIG. 1A is a plan view of a part of a semiconductor device according tothe first embodiment. Circuit elements such as a heterojunction bipolartransistor (HBT) 41 and a protection circuit 42 are formed on asubstrate 40 made of semi-insulating GaAs. A plurality of bonding pads140 are disposed so as to partially overlap the circuit elementconstituting the protection circuit 42.

FIG. 1B is a sectional view taken along dash-dotted line 1B-1B of FIG.1A. On the substrate 40, a buffer layer 101, a sub-collector layer 102,a collector layer 103, a base layer 104, an emitter layer 105, andcontact layers 106 and 107 are stacked in this order. The buffer layer101 is formed of undoped GaAs, and the sub-collector layer 102 and thecollector layer 103 are formed of n-type GaAs. The base layer 104 isformed of p-type GaAs. The emitter layer 105 is formed of n-type InGaP.The contact layers 106 and 107 are formed of n-type GaAs and n-typeInGaAs, respectively.

An element isolation region 108 is formed by implanting boron (B) intopartial regions of the buffer layer 101 and the sub-collector layer 102.The HBT 41 including the sub-collector layer 102, the collector layer103, the base layer 104, and the emitter layer 105 is formed on thesubstrate 40. A collector electrode 70 is formed on the sub-collectorlayer 102, and the collector electrode 70 is ohmically connected to thesub-collector layer 102. A base electrode 71 formed on the emitter layer105 penetrates through the emitter layer 105 and is ohmically connectedto the base layer 104. An emitter electrode 72 formed on the contactlayer 107 is ohmically connected to the contact layer 107.

A via-hole connecting pad 73 is formed on the element isolation region108 on the lateral side of the HBT 41. An interlayer insulating film 109is formed over the entire region of the substrate 40 so as to cover theHBT 41 and the via-hole connecting pad 73. For example, silicon nitride(SiN) is used for the interlayer insulating film 109. A plurality ofcontact holes are formed at predetermined positions of the interlayerinsulating film 109, for example, at positions where the collectorelectrode 70, the base electrode 71, the emitter electrode 72, and thevia-hole connecting pad 73 are disposed.

A wiring line 111 formed on the interlayer insulating film 109 connectsthe emitter electrode 72 and the via-hole connecting pad 73.Furthermore, a plurality of other wiring lines 111 are connected to thecollector electrode 70 and the base electrode 71.

An interlayer insulating film 112 is formed over the entire region ofthe substrate 40 so as to cover the wiring lines 111. An upperinterlayer insulating film 113 is further formed on the interlayerinsulating film 112. For example, SiN is used for the interlayerinsulating film 112, and a polyimide is used for the upper interlayerinsulating film 113. A protective film 121 is formed on the interlayerinsulating film 113. For example, SiN is used for the protective film121.

A via-hole 43 is formed from a back surface of the substrate 40 towardthe via-hole connecting pad 73. A back-surface electrode 45 formed onthe back surface of the substrate 40 is connected to the via-holeconnecting pad 73 through the via-hole 43.

FIG. 1C is a sectional view taken along dash-dotted lines 1C-1C of FIG.1A. A plurality of diodes 47 using the p-n junction of the collectorlayer 103 and the base layer 104 are formed on the substrate 40. Thediodes 47 are connected in series to thereby form the protection circuit42 (FIG. 1A).

A cathode electrode 80 formed on the sub-collector layer 102 of each ofthe diodes 47 is ohmically connected to the sub-collector layer 102. Ananode electrode 81 formed on the emitter layer 105 of each of the diodes47 penetrates through the emitter layer 105 and is ohmically connectedto the base layer 104. A via-hole connecting pad 74 is formed on theelement isolation region 108 on the lateral side of one of the diodes47.

The interlayer insulating film 109 is formed over the entire region ofthe substrate 40 so as to cover the diodes 47 and the via-holeconnecting pad 74. Contact holes are formed at predetermined positionsof the interlayer insulating film 109, for example, at positions wherethe cathode electrodes 80, the anode electrodes 81, and the via-holeconnecting pad 74 are disposed. A plurality of wiring lines 111 formedon the interlayer insulating film 109 connect the cathode electrodes 80and the anode electrodes 81 of the diodes 47 to thereby connect thediodes 47 in series. Another wiring line 111 connects the cathodeelectrode 80 at an end portion of a series circuit formed by the diodes47 and the via-hole connecting pad 74. The anode electrode 81 at an endportion of the series circuit formed by the diodes 47 is connected tothe collector electrode 70 of the HBT 41 (FIG. 1B) by another wiringline 111.

The interlayer insulating film 112 is formed over the entire region ofthe substrate 40 so as to cover the wiring lines 111. A first metal film120 for bonding pads is formed on the interlayer insulating film 112.The first metal film 120 is disposed at a position which partiallyoverlaps the plurality of diodes 47.

The protective film 121 is formed on the first metal film 120 and theinterlayer insulating film 112. A plurality of openings 122 for bondingare formed in the protective film 121. The openings 122 are disposed atpositions which are located inside the first metal film 120 in plan viewand which partially overlap the plurality of diodes 47.

A seed electrode layer 127 for plating is formed so as to cover thebottom surface and the side surface of each of the openings 122. Theseed electrode layer 127 extends to a frame-like region of the uppersurface of the protective film 121, the frame-like region surroundingthe opening 122. A second metal film 130 and a third metal film 131 forbonding pads are stacked on the seed electrode layer 127 in this order.The first metal film 120, the second metal film 130, and the third metalfilm 131 form bonding pads 140.

The metal material of the second metal film 130 is harder than the metalmaterial of the first metal film 120. The hardness of the metalmaterials can be defined by, for example, Vickers hardness, Young'smodulus, etc. The third metal film 131 is formed of the same metalmaterial as the first metal film 120. For example, the first metal film120 and the third metal film 131 are formed of gold (Au), and the secondmetal film 130 is formed of copper (Cu).

A via-hole 44 is formed to extend from the back surface of the substrate40 to the via-hole connecting pad 74. A back-surface electrode 45 formedon the back surface of the substrate 40 is connected to the via-holeconnecting pad 74 through the via-hole 44.

FIG. 2 is an equivalent circuit diagram of a power amplifier moduleincluding a semiconductor device according to the first embodiment. Thepower amplifier module includes an input matching circuit 520, asemiconductor device 500, and an output matching circuit 540.

The semiconductor device 500 includes an initial-stage HBT 502 and anoutput-stage HBT 41 that constitute a power amplifier circuit. An inputsignal is input to a bonding pad 501 for signal input. The input signalis input to a base electrode of the initial-stage HBT 502. The output ofthe initial-stage HBT 502 is input to the base electrode 71 (FIG. 1B) ofthe output-stage HBT 41.

Bias circuits 503 and 504 respectively supply the initial-stage HBT 502and the output-stage HBT 41 with a bias current. A protection circuit 42is connected between the collector and the emitter of the output-stageHBT 41. The protection circuit 42 includes a plurality of, for example10, diodes 47 connected in series. The collector of the output-stage HBT41 is connected to a plurality of bonding pads 140 serving as outputterminals.

A high-frequency signal is input to an input terminal 521 of the inputmatching circuit 520. A bonding pad 522 for signal output of the inputmatching circuit 520 is connected to the bonding pad 501 for signalinput of the semiconductor device 500 with a bonding wire 523therebetween.

The bonding pads 140 for signal output of the semiconductor device 500are connected to a plurality of bonding pads 541 for signal input of theoutput matching circuit 540 with a plurality of bonding wires 505therebetween. The high-frequency signal input to the input terminal 521of the input matching circuit 520 is amplified in the semiconductordevice 500 via the input matching circuit 520 and is output to an outputterminal 542 via the output matching circuit 540.

Advantageous Effects of First Embodiment

Next, a significantly advantageous effect of the first embodiment willbe described.

In the first embodiment, the protection circuit 42 (FIG. 1A) has afunction of protecting the HBT 41 (FIG. 1B) from electrostaticbreakdown, overvoltage breakdown, and load mismatch breakdown.Furthermore, in the first embodiment, since the protection circuit 42(FIG. 1A) and the bonding pads 140 are disposed so as to partiallyoverlap, it is not necessary to secure dedicated regions for arrangingthe bonding pads 140. With this structure, an increase in the chip areacan be suppressed.

Next, another advantageous effect of the first embodiment will bedescribed with reference to FIGS. 3A to 3D.

FIGS. 3A and 3B are schematic sectional views of a bonding pad 140 ofthe semiconductor device according to the first embodiment before wirebonding and during wire bonding, respectively.

A bonding wire 505 is supported by a capillary 510. For example, a gold(Au) wire is used as the bonding wire 505. A metal ball 506 is formed atan end of the bonding wire 505. When the capillary 510 is pressed ontothe bonding pad 140, and heat or ultrasonic waves are applied, a thirdmetal film 131 of the bonding pad 140 and the metal ball 506 aredeformed and bonded to each other. A relatively large stress isgenerated in the third metal film 131 along the outer periphery of themetal ball 506. A curve 511 illustrates the distribution of the stressapplied to the third metal film 131.

The stress generated in the third metal film 131 is transferred to aprotection circuit 42 through a second metal film 130, which isrelatively hard. Since the second metal film 130 functions as a stressdispersion sheet, the distribution of the stress generated in theprotection circuit 42 located below the second metal film 130 is gentlerthan the distribution of the stress generated in the third metal film131. A curve 512 illustrates the distribution of the stress generated inthe protection circuit 42.

FIGS. 3C and 3D are schematic sectional views of a bonding pad 143 of asemiconductor device according to a comparative example before wirebonding and during wire bonding, respectively. In the comparativeexample, the bonding pad 143 includes a single metal film made of thesame metal material as the first metal film 120 and the third metal film131 (FIGS. 3A and 3B). Since the bonding pad 143 is softer than thesecond metal film 130 (FIGS. 3A and 3B), the function of dispersing astress is low in the bonding pad 143. Accordingly, as illustrated in acurve 512, a large stress is generated in the protection circuit 42along the outer periphery of the metal ball 506. In the portion where alarge stress is generated, damage of a circuit element constituting theprotection circuit 42 easily occurs.

In the first embodiment, since the second metal film 130 of the bondingpad 140 functions as a stress dispersion sheet, damage of the protectioncircuit 42 is unlikely to occur during bonding. Furthermore, since ametal material that is softer than the second metal film 130 is used asthe third metal film 131, good electrical connection between the bondingwire 505 and the bonding pad 140 can be reliably established.

From the viewpoint of obtaining a stable contact interface, the firstmetal film 120 is preferably formed of the same metal material as thewiring line 111 (FIGS. 1B and 1C) disposed thereunder. When a circuitelement is formed by using a compound semiconductor, Au is typicallyused as the wiring line 111. Accordingly, Au is preferably used as thefirst metal film 120.

Next, metal materials suitable for the second metal film 130 of thebonding pad 140 will be described with reference to FIG. 4. In order toenhance the effect of the stress dispersion sheet, the second metal film130 is preferably made of a metal material that is harder than the firstmetal film 120. Herein, the term “hard” means that Vickers hardness ishigh or Young's modulus is high. Furthermore, from the viewpoint thatthe second metal film 130 forms a part of the bonding pad 140, a metalmaterial having a low electrical resistivity is preferably used.

FIG. 4 is a table listing Vickers hardness, Young's modulus, andelectrical resistivity of various metals. The values of Vickers hardnessshown in FIG. 4 are values measured with the same test force for all themetals.

When Au is used as the first metal film 120, the second metal film 130is preferably formed of a metal material having a higher Vickershardness or a higher Young's modulus than gold (Au). For example, ametal material having a Vickers hardness of higher than 30 HV or a metalmaterial having a Young's modulus of higher than 90 GPa is preferablyused as the second metal film 130. Examples of the metal material havinga higher Vickers hardness or Young's modulus than Au and having anelectrical resistivity that is not significantly high include copper(Cu), nickel (Ni), and molybdenum (Mo). The second metal film 130 ispreferably formed of Cu, Ni, or Mo.

When the bonding pad 140 has a large thickness enough to disperse thestress, the effect due to interposition of the hard second metal film130 (FIG. 1C) is small. A significant effect due to interposition of thesecond metal film 130 is obtained when the bonding pad 140 has athickness of 10 μm or less.

Modification of First Embodiment

In the first embodiment, the bonding pads 140 (FIG. 1A) partiallyoverlap the diodes 47 (FIG. 1C) that constitute the protection circuit42. Alternatively, the bonding pads 140 may overlap the HBT 41 (FIGS. 1Aand 1B). As illustrated in FIG. 2, circuit elements such as a capacitorand a resistive element are formed on the substrate 40 (FIGS. 1B and1C). The bonding pads 140 may partially overlap these circuit elements.Active elements other than HBTs, for example, field-effect transistorssuch as high electron mobility transistors (HEMTs) andmetal-semiconductor field-effect transistors (MESFETs) may be formed onthe substrate 40 made of a compound semiconductor. The bonding pads 140may partially overlap these active elements formed by using compoundsemiconductors.

FIG. 1A illustrates an example in which the number of the bonding pads140 is 3. However, the number of the bonding pads 140 is not limited to3, and at least one bonding pad 140 may be disposed. FIG. 2 illustratesan example in which the number of the output-stage HBT 41 is 1. However,in general, a plurality of HBTs that are connected in parallelconstitute the output stage.

A diffusion barrier layer may be disposed at an interface between thesecond metal film 130 (FIG. 1C) made of Cu and the third metal film 131(FIG. 1C) made of Au. For example, TiW or Ni can be used as thediffusion barrier layer.

In the first embodiment, a GaAs substrate is used as the substrate 40.Alternatively, other compound semiconductor substrates may be used. Forexample, an InP substrate may be used as the substrate 40.

In the first embodiment, for example, the first metal film 120 and thesecond metal film 130 have the same thickness. However, the thickness ofthe second metal film 130 is preferably equal to or larger than thethickness of the first metal film 120. By increasing the thickness ofthe second metal film 130, the function of the second metal film 130 asa stress dispersion sheet can be enhanced.

Second Embodiment

Next, a semiconductor device according to a second embodiment will bedescribed with reference to FIG. 5. Hereinafter, a description ofconfigurations common to the configurations of the semiconductor deviceaccording to the first embodiment is omitted.

FIG. 5 is a sectional view of a semiconductor device according to thesecond embodiment. In the first embodiment, as illustrated in FIG. 1C,the second metal film 130 and the third metal film 131 of the bondingpad 140 extend to the outside of the edge of the opening 122, and anouter peripheral portion thereof overlaps the protective film 121. Incontrast, in the second embodiment, a second metal film 130 and a thirdmetal film 131 are disposed inside an opening 122 in plan view.Specifically, the second metal film 130 and the third metal film 131 donot overlap a protective film 121. A seed electrode layer 127 is alsodisposed inside the opening 122 in plan view.

In the second embodiment, even when a load is applied to the secondmetal film 130 and the third metal film 131 of the bonding pad 140during bonding, the load is not applied to the protective film 121. Withthis structure, a mechanical stress applied to the protective film 121during bonding can be reduced. As a result, generation of cracks in theprotective film 121 can be suppressed.

Third Embodiment

Next, a semiconductor device according to a third embodiment will bedescribed with reference to FIG. 6. Hereinafter, a description ofconfigurations common to the configurations of the semiconductor deviceaccording to the second embodiment is omitted.

FIG. 6 is a sectional view of a semiconductor device according to thethird embodiment. In the second embodiment, side surfaces and an uppersurface of a multilayer structure including the seed electrode layer127, the second metal film 130, and the third metal film 131 (FIG. 5) ofthe bonding pad 140 are exposed. In the third embodiment, side surfacesand an upper surface of a multilayer structure including a seedelectrode layer 127, a second metal film 130, and a third metal film 131are covered with a fourth metal film 132. The fourth metal film 132further covers a region of an upper surface of a first metal film 120 inan opening 122, the region not having the seed electrode layer 127thereon. The fourth metal film 132 is formed of, for example, the samemetal material as the third metal film 131.

In the third embodiment, a contact surface of dissimilar metals of thesecond metal film 130 and the third metal film 131 is not exposed. Withthis structure, galvanic corrosion can be suppressed particularly in ahigh-humidity environment.

Next, a method for producing a semiconductor device according to thethird embodiment will be described with reference to FIGS. 7A and 7B toFIGS. 30A and 30B. The method for producing a semiconductor deviceaccording to the third embodiment can be applied also to the productionof the semiconductor devices according to the first embodiment and thesecond embodiment.

FIGS. 7A and 7B to FIGS. 30A and 30B are sectional views of asemiconductor device according to the third embodiment during itsproduction. In the drawings from FIGS. 7A and 7B to FIGS. 30A and 30B,figures suffixed with A correspond to sectional views taken alongdash-dotted line 1B-1B of FIG. 1A, and figures suffixed with Bcorrespond to sectional views taken along dash-dotted line 1C-1C of FIG.1A.

As illustrated in FIGS. 7A and 7B, a buffer layer 101, a sub-collectorlayer 102, a collector layer 103, a base layer 104, an emitter layer105, and contact layers 106 and 107 are sequentially epitaxially grownon a substrate 40 made of semi-insulating GaAs. For example, ametal-organic vapor phase epitaxy (MOVPE) can be used to form thesesemiconductor layers. The substrate 40 has as an upper surface having anoff-angle of 4° or less from a (100) plane.

Next, a description will be made of an example of the materials, thedoping concentrations, and the film thicknesses of the semiconductorlayers of the buffer layer 101 to the contact layer 107. The bufferlayer 101 is formed of undoped GaAs and has a film thickness of 0.1 μm.The sub-collector layer 102 is formed of n-type GaAs, has a dopingconcentration of Si of 5×10¹⁸ cm⁻³, the Si being an n-type dopant, andhas a film thickness of 0.6 μm. The collector layer 103 is formed ofn-type GaAs, has a doping concentration of Si of 1×10¹⁶ cm⁻³, and has afilm thickness of 1.0 μm. The base layer 104 is formed of p-type GaAs,has a doping concentration of C of 5×10¹⁹ cm⁻³, the C being a p-typedopant, and has a film thickness of 96 nm. The emitter layer 105 isformed of n-type InGaP, has a molar ratio of InP of 0.48, has a dopingconcentration of Si of 4×10¹⁷ cm⁻³, and has a film thickness of 35 nm.The contact layer 106 is formed of n-type GaAs, has a dopingconcentration of Si of 5×10¹⁸ cm⁻³, and has a film thickness of 50 nm.The upper contact layer 107 is formed of n-type InGaAs, has a molarratio of InAs of 0.5, has a doping concentration of Si of 1×10¹⁹ cm⁻³,and has a film thickness of 50 nm.

As illustrated in FIG. 8A, an emitter electrode 72 is formed on apredetermined region of the contact layer 107. The emitter electrode 72is not disposed on the section illustrated in FIG. 8B. The emitterelectrode 72 has a four-layer structure in which a Mo film having athickness of 10 nm, a Ti film having a thickness of 5 nm, a Pt filmhaving a thickness of 30 nm, and a Au film having a thickness of 200 nmare sequentially stacked from the substrate 40 side. The emitterelectrode 72 can be formed by vapor deposition and a lift-off method.

As illustrated in FIG. 9A, the contact layers 107 and 106 are processedto have a predetermined shape to form an emitter region 150. The emitterelectrode 72 is disposed on the emitter region 150. In the sectionillustrated in FIG. 9B, the contact layers 107 and 106 are removed toexpose the emitter layer 105. The contact layers 107 and 106 can beprocessed by photolithography and wet etching. This wet etching can beperformed by using, for example, an etchant prepared by mixingphosphoric acid, a hydrogen peroxide solution, and water. Specifically,for example, an etchant prepared by mixing phosphoric acid having aconcentration of 85% by weight, a hydrogen peroxide solution having aconcentration of 35% by weight, and water in a volume ratio of 1:2:40can be used. This etchant has a selectivity for selectively etching thecontact layers 107 and 106 made of GaAs without substantially etchingthe emitter layer 105 made of InGaP.

As illustrated in FIG. 10A, a base electrode 71 is formed on both sidesof the emitter region 150. The base electrode 71 is formed by forming ametal film on the emitter layer 105 by photolithography, vapordeposition, and a lift-off method and then performing sintering. Thus,the base electrode 71 penetrates through the emitter layer 105 and isohmically connected to the base layer 104. The metal film for formingthe base electrode 71 includes a Pt film having a thickness of 30 nm, aTi film having a thickness of 50 nm, a Pt film having a thickness of 50nm, and a Au film having a thickness of 200 nm which are sequentiallystacked from the substrate 40 side.

As illustrated in FIG. 10B, a plurality of anode electrodes 81 areformed at the same time when the base electrode 71 (FIG. 10A) is formed.The anode electrodes 81 also penetrate through the emitter layer 105 andare ohmically connected to the base layer 104.

As illustrated in FIGS. 11A and 11B, unnecessary portions of the emitterlayer 105 are removed by photolithography and wet etching. As a result,the base layer 104 is exposed. For example, hydrochloric acid can beused as an etchant. Hydrochloric acid has a selectivity for selectivelyetching the emitter layer 105 made of InGaP without substantiallyetching the base layer 104 made of GaAs.

As illustrated in FIGS. 12A and 12B, unnecessary portions of the baselayer 104 and the collector layer 103 are removed by using the etchingmask used in the etching of the emitter layer 105. As a result, thesub-collector layer 102 is exposed. For the etching of the base layer104 and the collector layer 103, the same etchant as the etchant used inthe etching of the contact layers 107 and 106 (FIGS. 9A and 9B) can beused. Stopping of the etching is performed by controlling the time.

As illustrated in FIGS. 13A and 13B, element isolation regions 108 forensuring electrical insulation between elements are formed. The elementisolation regions 108 can be formed by, for example, ion implantation ofboron into the sub-collector layer 102 and the buffer layer 101.

As illustrated in FIGS. 14A and 14B, a collector electrode 70 is formedon the sub-collector layer 102 of an HBT 41 (FIG. 1B), and cathodeelectrodes 80 are formed on the sub-collector layer 102 of diodes 47(FIG. 1B) at the same time. Furthermore, via-hole connecting pads 73 and74 are formed on the element isolation region 108. The collectorelectrode 70, the cathode electrodes 80, and the via-hole connectingpads 73 and 74 can be formed by using photolithography, vapordeposition, and a lift-off method. The collector electrode 70, thecathode electrodes 80, and the via-hole connecting pads 73 and 74include a AuGe film having a thickness of 60 nm, a Ni film having athickness of 10 nm, and a Au film having a thickness of 200 nm which aresequentially stacked from the substrate 40 side.

As illustrated in FIGS. 15A and 15B, an interlayer insulating film 109is deposited over the entire region of the substrate 40 so as to coverthe HBT 41, the diodes 47, and the via-hole connecting pads 73 and 74.The interlayer insulating film 109 is formed of, for example, SiN andhas a thickness of 100 nm. For example, chemical vapor deposition (CVD)can be used to deposit the interlayer insulating film 109.

As illustrated in FIGS. 16A and 16B, a plurality of openings 110 areformed at predetermined positions of the interlayer insulating film 109.The openings 110 are disposed inside the collector electrode 70, thebase electrode 71, the emitter electrode 72, the cathode electrodes 80,the anode electrodes 81, and the via-hole connecting pads 73 and 74 inplan view and expose partial regions of the upper surfaces of these. Theopenings 110 can be formed by using photolithography and dry etching.

As illustrated in FIGS. 17A and 17B, a plurality of wiring lines 111 areformed on the interlayer insulating film 109. The wiring lines 111 areformed of, for example, Au and each have a thickness of 1 μm. The wiringlines 111 can be formed by using photolithography, vapor deposition, anda lift-off method. One of the wiring lines 111 connects the emitterelectrode 72 and the via-hole connecting pad 73. Another one of thewiring lines 111 connects the cathode electrode 80 of one diode 47 andthe via-hole connecting pad 74. Another one of the wiring lines 111connects the anode electrode 81 of one diode 47 and a cathode electrode80 of a diode 47 adjacent to the one diode 47. Furthermore, the baseelectrode 71 and the collector electrode 70 are each also connected tothe corresponding wiring line 111.

As illustrated in FIG. 18A, an interlayer insulating film 112 is formedover the entire region of the substrate 40 so as to cover the wiringlines 111. The interlayer insulating film 112 is formed of, for example,SiN. Furthermore, an interlayer insulating film 113 is formed on theinterlayer insulating film 112. The interlayer insulating film 113 isformed by, for example, applying a polyimide to form a polyimide filmhaving a thickness of 1.8 μm, and then planarizing the surface of thepolyimide film. Subsequently, the interlayer insulating film 113 inregions where bonding pads 140 (FIG. 1A and FIG. 6) are to be disposedis removed. In the section illustrated in FIG. 18B, the interlayerinsulating film 113 is removed, and the interlayer insulating film 112made of SiN is exposed.

As illustrated in FIG. 19B, a first metal film 120 is formed on theinterlayer insulating film 112. The first metal film 120 is disposed soas to partially overlap the plurality of diodes 47. The first metal film120 is not formed in the section illustrated in FIG. 19A. The firstmetal film 120 is formed of, for example, Au and has a thickness of 2μm. The first metal film 120 can be formed by using, for example,photolithography, vapor deposition, and a lift-off method. A secondwiring line layer is formed in another region of the substrate 40 at thesame time when the first metal film 120 is formed.

A protective film 121 made of SiN is formed over the entire region ofthe substrate 40 so as to cover the first metal film 120 illustrated inFIG. 19B. In the section illustrated in FIG. 19A, the protective film121 is formed on the interlayer insulating film 113. The protective film121 has a thickness of, for example, 500 nm. The protective film 121 canbe formed by using, for example, a CVD method. A resist film 160 isformed on the protective film 121, and openings 161 are formed inregions where bonding pads 140 (FIG. 1A and FIG. 6) are to be formed.

As illustrated in FIGS. 20A and 20B, the protective film 121 is etchedby using the resist film 160 as an etching mask to form openings 122 inthe protective film 121. The first metal film 120 is exposed inside theopenings 122. For example, dry etching can be used for etching theprotective film 121. After the openings 122 are formed in the protectivefilm 121, the resist film 160 is removed.

As illustrated in FIGS. 21A and 21B, a seed electrode layer 127 forplating is formed over the entire region of the substrate 40 so as tocover the upper surface of the protective film 121 and the side surfacesand bottom surfaces of the openings 122. The seed electrode layer 127includes, for example, two layers of a TiW film having a thickness of0.1 μm and a Cu film having a thickness of 0.1 μm. The seed electrodelayer 127 can be formed by using, for example, a sputtering method.

As illustrated in FIGS. 22A and 22B, a resist film 163 is formed on theseed electrode layer 127. Openings 164 are formed by photolithography inregions where a second metal film 130 and a third metal film 131 (FIG.6) are to be formed. The seed electrode layer 127 is exposed in theopenings 164.

As illustrated in FIGS. 23A and 23B, the second metal film 130 and thethird metal film 131 are sequentially deposited by electroplating on theseed electrode layer 127 in the openings 164. The second metal film 130is formed of, for example, Cu and has a thickness of 2 μm. The thirdmetal film 131 is formed of, for example, Au and has a thickness of 1μm. The upper surface of the third metal film 131 is lower than theupper surface of the resist film 163. After the formation of the secondmetal film 130 and the third metal film 131, the resist film 163 isremoved.

As illustrated in FIGS. 24A and 24B, the seed electrode layer 127exposed in regions where the second metal film 130 is not formed isremoved by etching. In the section illustrated in FIG. 24A, theprotective film 121 is exposed. In the section illustrated in FIG. 24B,the protective film 121 and the first metal film 120 located in theopenings 122 formed in the protective film 121 are exposed.

As illustrated in FIG. 25B, a fourth metal film 132 is formed on theside surfaces and the upper surface of a stacked structure including theseed electrode layer 127, the second metal film 130, and the third metalfilm 131, and the upper surface of the first metal film 120 exposed inthe openings 122. The fourth metal film 132 can be formed by using, forexample, electroless plating. The fourth metal film 132 is formed of,for example, Au and has a thickness of 0.1 μm. The Au is not depositedon the upper surface of the protective film 121 illustrated in FIGS. 25Aand 25B.

As illustrated in FIGS. 26A and 26B, a front surface of the substrate 40(the surface on which the protective film 121 is formed) is disposed soas to face a sapphire substrate 167, and the substrate 40 is attached tothe sapphire substrate 167 with wax 166 therebetween.

As illustrated in FIGS. 27A and 27B, the substrate 40 is ground from thebackside thereof to thereby reduce the thickness to 75 μm.

As illustrated in FIGS. 28A and 28B, a resist film 170 is formed on aback surface of the substrate 40 having the reduced thickness. Openings171 and 172 are formed in the resist film 170 by photolithography. Theopenings 171 and 172 are formed at positions corresponding to thevia-hole connecting pads 73 and 74, respectively.

As illustrated in FIGS. 29A and 29B, the substrate 40 and the elementisolation region 108 are etched by using the resist film 170 as anetching mask. As a result, via-holes 43 and 44 penetrating through thesubstrate 40 and the element isolation region 108 are formed, and thevia-hole connecting pads 73 and 74 are respectively exposed on thebottom surfaces thereof. Anisotropic dry etching can be used for etchingthe substrate 40 and the element isolation region 108. After theexposure of the via-hole connecting pads 73 and 74, the resist film 170is removed.

As illustrated in FIGS. 30A and 30B, a back-surface electrode 45 isformed on the back surface of the substrate 40, and the side surfacesand the bottom surfaces of the via-holes 43 and 44. The back-surfaceelectrode 45 can be formed by depositing palladium by electrolessplating and then depositing Au by electroplating. The back-surfaceelectrode 45 has a thickness of 4 μm. After the formation of theback-surface electrode 45, the wax 166 and the sapphire substrate 167are removed from the substrate 40.

As illustrated in FIGS. 31A and 31B, the fourth metal film 132, which isthe outermost layer of the bonding pads 140, and the protective film 121are exposed. Subsequently, for example, dicing is performed so that thesubstrate 40 is divided into chips. Thus, the semiconductor device iscompleted.

Fourth Embodiment

Next, a semiconductor device according to a fourth embodiment will bedescribed with reference to FIG. 32A. Hereinafter, a description ofconfigurations common to the configurations of the first embodiment,which have been described with reference to FIGS. 1A to 4, is omitted.

FIG. 32A is a sectional view of a semiconductor device according to thefourth embodiment. In the first embodiment, the second metal film 130(FIG. 1C) that forms the bonding pads 140 is constituted by a singlelayer made of a metal material harder than the first metal film 120. Inthe fourth embodiment, the second metal film 130 has a two-layerstructure including a lower metal film 130A and an upper metal film 130Bthat are made of different metal materials. The lower metal film 130Aand the upper metal film 130B are formed of metal materials harder thanthe first metal film 120. For example, two metals selected from threemetals of Cu, Ni, and Mo are preferably used as the lower metal film130A and the upper metal film 130B.

Next, semiconductor devices according to modifications of the fourthembodiment will be described with reference to FIGS. 32B and 32C.

FIGS. 32B and 32C are each a sectional view of a semiconductor deviceaccording to a modification of the fourth embodiment. In themodification illustrated in FIG. 32B, a second metal film 130 and athird metal film 131 are disposed inside an opening 122 provided in aprotective film 121, as in the bonding pad 140 (FIG. 5) of thesemiconductor device according to the second embodiment. In themodification illustrated in FIG. 32C, a stacked structure including asecond metal film 130 and a third metal film 131, and an upper surfaceof the first metal film 120 located on the periphery of the stackedstructure are covered with a fourth metal film 132, as in the bondingpad 140 (FIG. 6) of the semiconductor device according to the thirdembodiment. Also in the modifications illustrated in FIGS. 32B and 32C,the second metal film 130 includes two layers of a lower metal film 130Aand an upper metal film 130B.

In the fourth embodiment and the modifications of the fourth embodiment,a plurality of materials can be combined such that the second metal film130 has desired hardness as a whole, and an increase in the electricalresistivity is suppressed. As a result, semiconductor devices havinghigher reliability are provided.

Fifth Embodiment

Next, a semiconductor device according to a fifth embodiment will bedescribed with reference to FIGS. 33A to 34C. Hereinafter, a descriptionof configurations common to the configurations of the first embodimentto the fourth embodiment is omitted. In the first embodiment to thefourth embodiment, the indices of crystal plane of the upper surface ofthe substrate 40 (FIGS. 1B, 1C, etc.), and the planar shape and thecrystal orientation of a pattern of each semiconductor layer of thediodes 47 (FIG. 1C, etc.) are not limited. In the fifth embodiment, theindices of crystal plane of the upper surface of the substrate 40 arespecified, and planar shapes and preferred crystal orientations ofpatterns of semiconductor layers are limited.

First, anisotropic etching characteristics due to wet etching will bedescribed with reference to FIGS. 33A to 33E.

FIG. 33A is a plan view of a regular octagonal mesa formed, byanisotropic etching, on a GaAs substrate having a (100) plane as anupper surface. One side of the regular octagon is parallel to the [011]direction. FIGS. 33B, 33C, 33D, and 33E are sectional views taken alongdash-dotted lines 33B-33B, 33C-33C, 33D-33D, and 33E-33E of FIG. 33A,respectively. A weakly acidic or weakly alkaline etchant can be used asan etchant for performing the anisotropic etching.

In the section illustrated in FIG. 33B, an inverted mesa portion iseasily formed in which the (111)A plane appears on a side surfaceoriented in the [01-1] direction and a side surface oriented in theopposite direction thereof. For example, an upper layer portion of amesa structure has an inverted mesa shape, and a lower layer portionthereof has a normal mesa shape. An eave-shaped portion protruding in alateral direction is formed on the inverted mesa portion. Herein, theminus sign attached to an element of the Miller indices denotes that anoverbar is attached to the element. In the section illustrated in FIG.33C, the (111)B plane easily appears on a side surface oriented in the[0-1-1] direction and a side surface oriented in the opposite directionthereof, and the side surfaces form gently inclined surfaces. Thesection illustrated in FIG. 33D forms a shape in which a side surfaceoriented in the [00-1] direction and a side surface oriented in theopposite direction thereof rise substantially perpendicularly. Thesection illustrated in FIG. 33E forms a shape in which a side surfaceoriented in the [0-10] direction and a side surface oriented in theopposite direction thereof rise substantially perpendicularly.

Anisotropic etching characteristics illustrated in each of FIGS. 33B to33E are the same as the characteristics when a compound semiconductorthat is epitaxially grown on a GaAs substrate having a (100) plane as anupper surface is subjected to anisotropic etching. Similarcharacteristics are obtained, besides the case of a GaAs substrate, alsoin the case where a compound semiconductor substrate having a zincblendcrystal structure and a compound semiconductor layer that is epitaxiallygrown on the substrate are subjected to anisotropic etching.

FIG. 34A is a plan view of one diode 47 of a semiconductor deviceaccording to the fifth embodiment. FIGS. 34B and 34C are sectional viewstaken along dash-dotted lines 34B-34B and 34C-34C of FIG. 34A,respectively. A substrate 40 has an upper surface having indices ofcrystal plane of (100). The substrate 40 may be a substrate having anupper surface that is a crystal plane having an off-angle of 4° or lessfrom a (100) plane.

A collector layer 103, a base layer 104, and an emitter layer 105 arestacked on a sub-collector layer 102 surrounded by an element isolationregion 108. In plan view, the emitter layer 105 is disposed inside thecollector layer 103. A cathode electrode 80 formed on the sub-collectorlayer 102 surrounds the collector layer 103 from three directions of the[011] direction, the [0-11] direction, and the [0-1-1] direction. Ananode electrode 81 formed on the emitter layer 105 penetrates throughthe emitter layer 105 and is ohmically connected to the base layer 104.

A plurality of diodes 47 (FIG. 1C) are arranged in parallel in the[01-1] direction. This arrangement direction corresponds to an easydirection of cleavage of the substrate 40 made of GaAs. In other words,the diodes 47 are arranged in a direction parallel to the easy directionof cleavage.

The emitter layer 105, the base layer 104, and the collector layer 103are patterned by performing anisotropic etching using the same etchingmask (refer to FIGS. 11B and 12B). The etching mask has a rectangularplanar shape and has a long side parallel to the [01-1] direction and ashort side parallel to the [011] direction.

As illustrated in FIG. 34B, in a section perpendicular to the [0-1-1]direction, an upper layer portion of a mesa including the collectorlayer 103, the base layer 104, and the emitter layer 105 has an invertedmesa shape to form eave-shaped portions 83. The emitter layer 105illustrated in FIG. 34A corresponds to an outer peripheral line of theupper surface thereof.

As illustrated in FIG. 34C, in a section perpendicular to the [0-11]direction, the mesa including the collector layer 103, the base layer104, and the emitter layer 105 has a normal mesa shape to form gentlyinclined surfaces. The collector layer 103 illustrated in FIG. 34Aillustrates an outer peripheral line of the bottom surface thereof.

Next, a significantly advantageous effect of the fifth embodiment willbe described. During bonding, a load applied to the diodes 47 easilycauses breakage in the eave-shaped portions 83. In the fifth embodiment,in the outer peripheral line of the emitter layer 105 (FIG. 34A), thesides which are parallel to the [011] direction and on which theeave-shaped portions 83 are formed are shorter than the sides which areparallel to the [01-1] direction and on which the gently inclinedsurfaces are formed. Thus, regions where the eave-shaped portions 83 areformed are limited to a narrow range. Accordingly, breakage due to aload during bonding is unlikely to occur.

FIGS. 34A to 34C illustrate an example in which the anode electrode 81penetrates through the emitter layer 105 and is ohmically connected tothe base layer 104. Alternatively, the emitter layer 105 may be removed,and the anode electrode 81 may be directly formed on the base layer 104.In this case, an upper layer portion of the base layer 104 and collectorlayer 103 form the eave-shaped portions 83.

To enhance the effect of suppressing breakage of the eave-shapedportions 83, on the upper surface of the mesa portion having the anodeelectrode 81 thereon, the length of the short side is preferably ½ orless of the length of the long side.

Sixth Embodiment

Next, a semiconductor device according to a sixth embodiment will bedescribed with reference to FIGS. 35A to 35C. Hereinafter, a descriptionof configurations common to the configurations of the semiconductordevice according to the fifth embodiment is omitted.

FIG. 35A is a plan view of one diode 47 of a semiconductor deviceaccording to the sixth embodiment. FIGS. 35B and 35C are sectional viewstaken along dash-dotted lines 35B-35B and 35C-35C of FIG. 35A,respectively. In the fifth embodiment, the emitter layer 105 and thecollector layer 103 (FIG. 34A) each have a rectangular planar shape. Inthe sixth embodiment, planar shapes of an emitter layer 105 and acollector layer 103 are each a polygon including sides parallel to the[01-1] direction, sides parallel to the [001] direction, and sidesparallel to the [010] direction, for example, a parallel hexagon.

As illustrated in FIG. 35B, in a section that perpendicularly intersectsa side parallel to the [01-1] direction, a mesa including a collectorlayer 103, a base layer 104, and an emitter layer 105 has a normal mesashape as in the section illustrated in FIG. 34C of the fifth embodiment.As illustrated in FIG. 35A, the bottom surface of the collector layer103 extends in the [011] direction and the [0-1-1] direction withrespect to the bottom surface of the emitter layer 105.

As illustrated in FIG. 35C, a section that intersects a side parallel tothe [010] direction forms a shape in which a side surface of the mesaincluding the collector layer 103, the base layer 104, and the emitterlayer 105 rises perpendicularly. Similarly, a section that intersects aside parallel to the [001] direction also forms such a shape.

In the sixth embodiment, the mesa including the collector layer 103, thebase layer 104, and the emitter layer 105 of the diode 47 has noeave-shaped portions. This structure provides the advantageous effectthat breakage due to a load during bonding is unlikely to occur.

Seventh Embodiment

Next, a semiconductor device according to a seventh embodiment will bedescribed with reference to FIGS. 36A and 36B. Hereinafter, adescription of configurations common to the configurations of thesemiconductor device according to the sixth embodiment is omitted.

FIG. 36A is a plan view of one diode 47 of a semiconductor deviceaccording to the seventh embodiment. In the sixth embodiment, theemitter layer 105 has a parallel hexagonal planar shape (FIG. 35A). Inthe seventh embodiment, an emitter layer 105 has a square planar shapeincluding sides parallel to the [010] direction and sides parallel tothe direction.

FIG. 36B is a sectional view taken along dash-dotted line 36B-36B ofFIG. 36A. A section that intersects a side parallel to the [010]direction has a shape in which a side surface of a mesa including acollector layer 103, a base layer 104, and an emitter layer 105 risesperpendicularly. Similarly, a section that intersects a side parallel tothe direction also forms such a shape.

As described above, in the seventh embodiment, all of the side surfacesof the mesa that includes the collector layer 103, the base layer 104,and the emitter layer 105 rise perpendicularly, and no eave-shapedportions are formed. This structure provides the advantageous effectthat breakage due to a load during bonding is unlikely to occur.

Eighth Embodiment

A power amplifier module according to an eighth embodiment will bedescribed with reference to FIG. 37. This power amplifier moduleincludes the semiconductor device according to any of the firstembodiment to the seventh embodiment.

FIG. 37 is a sectional view of a power amplifier module according to theeighth embodiment. A semiconductor device 500, a silicon semiconductorchip 230, and a plurality of other surface mount elements are mounted ona printed circuit board 200. The semiconductor device according to anyof the first embodiment to the seventh embodiment is used as thesemiconductor device 500. The silicon semiconductor chip 230 includes,for example, a CMOS circuit formed therein.

A back-surface electrode 45 of the semiconductor device 500 is fixed andelectrically connected to a die pad 201 of the printed circuit board 200with a conductive adhesive 221 therebetween. The silicon semiconductorchip 230 is fixed to a die pad 202 of the printed circuit board 200 witha conductive adhesive 231 therebetween. A Au film is formed on theoutermost surface of each of the die pads 201 and 202. For example, anadhesive containing, as a main component, an epoxy resin in which silver(Ag) fine particles are dispersed can be used as the conductiveadhesives 221 and 231. Besides an epoxy resin, an acrylic resin, abismaleimide resin, a butadiene resin, a silicone resin, or a resinmixture prepared by mixing these resins may be used.

A plurality of bonding pads 140 of the semiconductor device 500 areconnected to a bonding pad 203 of the printed circuit board 200 with aplurality of bonding wires 225 therebetween. For example, a Au wirehaving a diameter of 20 μm can be used as the bonding wires 225. Thebonding wires 225 are bonded to the bonding pads 140 and 203 by using awire bonder.

A plurality of bonding pads 232 of the silicon semiconductor chip 230are connected to a plurality of bonding pads 204 of the printed circuitboard 200 with a plurality of bonding wires 235 therebetween. Thesemiconductor device 500, the silicon semiconductor chip 230, the othersurface mount elements, and the bonding wires 225 and 235 are sealedwith a sealing cured resin 240.

In the eighth embodiment, since the semiconductor device according toany of the first embodiment to the seventh embodiment is used as thesemiconductor device 500, miniaturization of the semiconductor device500 can be realized. Furthermore, damage of the semiconductor device 500during bonding can be suppressed. Accordingly, a highly reliable poweramplifier module is provided.

The embodiments described above are exemplary, and, needless to say, apartial replacement or combination of configurations described indifferent embodiments is possible. The same or similar advantageouseffects achieved by the same or similar configurations in a plurality ofembodiments will not be mentioned in each of the embodiments.Furthermore, the present disclosure is not limited to the embodimentsdescribed above. For example, it is obvious for those skilled in the artthat various modifications, improvements, combinations, and the like canbe made.

While preferred embodiments of the disclosure have been described above,it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the disclosure. The scope of the disclosure, therefore, isto be determined solely by the following claims.

What is claimed is:
 1. A semiconductor device comprising: a circuitelement formed on a substrate made of a compound semiconductor; and abonding pad disposed on the circuit element so as to at least partiallyoverlap the circuit element, wherein the bonding pad includes a firstmetal film and a second metal film formed on the first metal film, ametal material of the second metal film has a higher Young's modulusthan a metal material of the first metal film, the second metal filmincludes two layers made of different metal materials, the bonding padincludes a third metal film formed on the second metal film, and thethird metal film is formed of the same metal material as the first metalfilm.
 2. The semiconductor device according to claim 1, furthercomprising: a seed electrode layer formed between the first metal filmand the second metal film.
 3. The semiconductor device according toclaim 1, further comprising: a protective film that is formed on thefirst metal film and has an opening disposed inside the first metal filmin plan view, and wherein the second metal film and the third metal filmare disposed inside the opening in plan view.
 4. The semiconductordevice according to claim 3, wherein: the bonding pad includes a fourthmetal film that is formed of the same metal material as the first metalfilm and a portion of the fourth metal film covers a region of an uppersurface of the first metal film inside the opening, the region not beingcovered with the second metal film, a side surface of the second metalfilm, and a side surface and an upper surface of the third metal film.5. The semiconductor device according to claim 1, wherein: the firstmetal film is formed of Au, and at least one of the two layers of thesecond metal film is made of at least one metal material selected fromthe group consisting of Cu, Ni, and Mo.
 6. The semiconductor deviceaccording to claim 5, wherein: the two layers of the second metal filmare made of different metal materials selected from the group consistingof Cu, Ni, and Mo.
 7. The semiconductor device according to claim 1,wherein: the circuit element is one element selected from the groupconsisting of a heterojunction bipolar transistor, a field-effecttransistor, a diode, a capacitor, and a resistive element.
 8. Thesemiconductor device according to claim 1, wherein: a thickness of thesecond metal film is equal to or larger than a thickness of the firstmetal film.
 9. The semiconductor device according to claim 1, wherein:the compound semiconductor of the substrate has a zincblend crystalstructure, and the substrate has an upper surface having an off-angle of4° or less from a (100) plane.
 10. The semiconductor device according toclaim 9, wherein: the circuit element includes a mesa structureincluding a semiconductor layer that is epitaxially grown from thesubstrate, an upper surface of the mesa structure has a rectangular orsquare planar shape including a side parallel to a [01-1] direction anda side parallel to a [011] direction, and the side parallel to the [011]direction is shorter than the side parallel to the [01-1] direction whenthe mesa structure has a rectangular shape.
 11. The semiconductor deviceaccording to claim 9, wherein: the circuit element includes a mesastructure including a semiconductor layer that is epitaxially grown fromthe substrate, and an upper surface of the mesa structure has apolygonal planar shape including a side parallel to a [001] directionand a side parallel to a [010] direction.
 12. The semiconductor deviceaccording to claim 11, wherein the upper surface of the mesa structurehas a parallel hexagonal planar shape further including a side parallelto a [01-1] direction.
 13. A power amplifier module comprising: asemiconductor device including a power amplifier circuit that includes aheterojunction bipolar transistor formed on a substrate made of acompound semiconductor, a protection circuit connected between anemitter and a collector of the heterojunction bipolar transistor, and abonding pad functioning as an output terminal of the power amplifiercircuit; a printed circuit board on which the semiconductor device ismounted; and a bonding wire that is bonded to the bonding pad to connectthe bonding pad to a wiring line of the printed circuit board, whereinat least one circuit element that forms the power amplifier circuit, theprotection circuit and the bonding pad are disposed so as to partiallyoverlap, the bonding pad includes at least two layers of a first metalfilm and a second metal film, and a metal material of the second metalfilm has a higher Young's modulus than a metal material of the firstmetal film, the second metal film includes two layers made of differentmetal materials, the bonding pad includes a third metal film formed onthe second metal film, and the third metal film is formed of the samemetal material as the first metal film.
 14. The power amplifier moduleaccording to claim 13, wherein: the semiconductor device furtherincludes a seed electrode layer formed between the first metal film andthe second metal film.
 15. The power amplifier module according to claim13, wherein: the semiconductor further includes a protective film thatis formed on the first metal film and has an opening disposed inside thefirst metal film in plan view, and the second metal film and the thirdmetal film are disposed inside the opening in plan view.
 16. The poweramplifier module according to claim 15, wherein: the bonding padincludes a fourth metal film that is formed of the same metal materialas the first metal film and a portion of the fourth metal film covers aregion of an upper surface of the first metal film inside the opening,the region not being covered with the second metal film, a side surfaceof the second metal film, and a side surface and an upper surface of thethird metal film.
 17. A semiconductor device comprising: a circuitelement formed on a substrate made of a compound semiconductor; and abonding pad disposed on the circuit element so as to at least partiallyoverlap the circuit element, wherein the bonding pad includes a firstmetal film and a second metal film formed on the first metal film, ametal material of the second metal film has a higher Young's modulusthan a metal material of the first metal film, a thickness of the secondmetal film is equal to or larger than a thickness of the first metalfilm, and the bonding pad includes a third metal film formed on thesecond metal film, and the third metal film is formed of the same metalmaterial as the first metal film.
 18. The semiconductor device accordingto claim 17, further comprising: a seed electrode layer formed betweenthe first metal film and the second metal film.
 19. The semiconductordevice according to claim 17, further comprising: a protective film thatis formed on the first metal film and has an opening disposed inside thefirst metal film in plan view, and wherein the second metal film and thethird metal film are disposed inside the opening in plan view.
 20. Thesemiconductor device according to claim 19, wherein: the bonding padincludes a fourth metal film that is formed of the same metal materialas the first metal film and a portion of the fourth metal film covers aregion of an upper surface of the first metal film inside the opening,the region not being covered with the second metal film, a side surfaceof the second metal film, and a side surface and an upper surface of thethird metal film.